1. Field of the Invention
The present invention relates to compensating circuitry for compensating erased periodic signal data and a compensating method therefor, and is applicable to, e.g. the compensation of the erasure of a speech signal.
2. Description of the Background Art
While speech communication over Internet or similar communication network is extensively used today, it is likely that speech sent over a network is partly erased or lost, resulting in the degradation speech quality. To improve degraded speech quality, a method taught in ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) Recommendation G.711 Appendix I is available.
In accordance with the method taught in the above document, a coded speech signal arrived over a network is decoded by a speech decoder and then input to a compensating circuitry. The compensating circuitry monitors the input decoded speech signal on a speech frame basis, which is the unit of speech signal decoding, and executes compensation every time the erasure of speech occurs. More specifically, when any speech is missing, the compensating circuitry determines a period or waveform frequency around the time when an erasure has occurred on the basis of speech data stored in, e.g. a memory included in the circuitry, and received just before the above time. Subsequently, the compensating circuitry reads out the speech data stored in the memory, and substitutes the data for a frame which the erasure is associated with and requires speech signal substitution, such that the start phase of the frame coincides with the end phase of the immediately preceding frame to thereby maintain continuity in waveform period.
The memory of the compensating circuitry has a storage capacity large enough to store speech data over, e.g. up to three consecutive waveform periods, so that an undesirable tone ascribable to a single continuous waveform can be obviated by use of the three waveform periods of speech data. Should only one waveform period of speech data be saved, it would cause unnecessary tones to generate when repeatedly used for substitution.
However, saving up to three waveform periods of speech data for the compensation of an erasure is not practicable without scaling up the memory and access configuration thereof and therefore the entire compensating circuitry. In addition, when erasure frames occur successively, the section for use in forming substitution data under speech was expanded by a multiple of the waveform period. Therefore, when erasure frames come successively, data under speech available for forming substitution data would resultantly be obtained from the longer section. Accordingly, the naturality in tonal fluctuation of a substituted speech may be spoiled.